Elsevier

Marine Chemistry

Volume 82, Issues 1–2, June 2003, Pages 115-123
Marine Chemistry

Recalcitrant dissolved organic matter in the ocean: major contribution of small amphiphilics

https://doi.org/10.1016/S0304-4203(03)00068-9Get rights and content

Abstract

For the bulk of dissolved organic carbon (DOC) in the ocean, chemical structure and basic biochemical parameters are largely unknown. To reveal new structural and physicochemical information on DOC, we applied gel permeation chromatography (GPC) in conjunction with a high-sensitivity DOC detector on samples from the Arctic Rivers and Ocean. Carbon-normalized data on molecular size, polarity, charge and aromaticity were obtained for marine-derived and terrigenous refractory compounds in the ocean. The main fraction (60–70%) of riverine DOC exhibited characteristic features of terrigenous humics in terms of aromaticity and molecular mass. Molar UV absorption coefficients (εOC at 254 nm) of 72–78 m2·mol−1 indicated high aromaticity and nominal molecular masses (Mn) were 870–1050 g·mol−1. The remainder was composed mainly of subunits of humics, and of diagenetically fresh biopolymers. In the coastal mixing zone, the hydrodynamic diameter of humics decreased probably due to intramolecular contraction or coiling induced by increases of ion strength and concentration of divalent ions. Aromaticity and the proportion of chromatographic DOC fractions, on the other hand, behaved conservatively, which indicates compositional stability of terrigenous DOC during mixing in the Arctic Ocean. In the deep sea, a main fraction (∼60%) of DOC showed characteristic features of aquatic fulvic acids and their subunits, with low molecular masses (Mn∼550 g·mol−1) and low aromaticities (εOC∼13 m2·mol−1). The remainder (∼40% of DOC) was composed of aliphatic and very small (<300 g·mol−1) neutral molecules, largely with hydrophobic functional groups. These molecules have escaped from the analytical window so far because they cannot be isolated via ultrafiltration or XAD resins and do not absorb UV light to be seen by conventional detection. Therefore, a considerable part of dissolved organic matter has very different chemical structures and physicochemical features than previously thought. Most mechanisms that have been suggested to describe the formation of recalcitrant organic matter in the ocean cannot be applied to a major fraction of DOC in the Arctic Ocean.

Introduction

Dissolved organic matter (DOM) in the ocean comprises a similar mass of carbon as atmospheric CO2 or the global biomass Druffel et al., 1992, Hedges et al., 1997. Recalcitrant organic compounds in the ocean exhibit an average age of several thousand years Williams and Druffel, 1987, Bauer et al., 1992 and sequester carbon from active cycles, thereby influencing atmospheric CO2 concentrations. Source, diagenesis and preservation mechanisms of DOM are largely unknown and matters of controversial discussions. Recent studies highlight the role of the continents and their margins as a considerable source of refractory and old DOM to the ocean Bauer and Druffel, 1998, Raymond and Bauer, 2001. The fate of terrigenous humic substances in the ocean is still unclear although the flux from the continents would be sufficient alone to explain turnover and concentration of dissolved organic carbon (DOC) in the world's oceans (Hedges et al., 1997). Recognizable terrigenous compounds are not detected in deep-sea DOM in quantities that correspond to their inputs Meyers-Schulte and Hedges, 1986, Opsahl and Benner, 1997, Hernes and Benner, 2002. Despite considerable efforts in the last decades, the percentage of DOC that can be chemically characterized decreases rapidly down ocean water columns to <30% at depth Carlson et al., 1985a, Benner, 1998. Even basic biochemical parameters are largely unknown for bulk DOC.

Gel permeation chromatography with simultaneous detection of organic carbon and UV absorption (GPC-OCD) provides information of molecular size distribution, polarity, charge and aromaticity for bulk DOC Huber and Frimmel, 1996, Specht and Frimmel, 2000. An enormous advantage of this method is that aquatic samples can directly be analyzed without any pretreatment, thereby avoiding artifacts introduced by extraction, hydrolysis or other procedural steps required by most analytical methods. This method is well established for characterizing DOC in freshwater systems (e.g. DeNobili and Chen, 1999), and it was demonstrated that this method could also be applied on marine samples (Huber and Frimmel, 1994).

The Arctic Ocean is well suited to study DOC and especially refractory compounds of marine and terrigenous origin in the ocean because it has the highest terrestrial input in terms of freshwater and organic matter on a volume basis, compared to the world's oceans. Due to the ice cover and the low-temperature, primary production and microbial activity are low. DOM is therefore composed largely of two endmembers that mix conservatively in the Arctic Ocean: refractory compounds of allochthonous marine and terrestrial origin (Dittmar et al., 2001).

The objective of this study was to determine the physicochemical characteristics of bulk DOC in the ocean from marine and terrestrial origins, which are essential prerequisites for diagenetic models of DOM in the ocean. Refractory DOC from the two endmembers, rivers and deep sea, and at different stages of mixing in the Arctic Ocean was chemically characterized by GPC-OCD.

Section snippets

Materials and methods

Samples were collected with the RV “Akademik Fedorov” in June–July 1994 (rivers and nearshore waters) and the RV “Polarstern” from July to September 1995 (Laptev Sea and Amundsen Basin). The Siberian rivers, Yenisey, Olenëk and Moroyyakha, were sampled at the upper part of their estuaries (salinity∼0) and at adjacent nearshore waters from the surface. The Laptev Sea and Amundsen Basin were sampled at standard CTD depths. All samples were taken from Niskin bottles fitted to a CTD rosette system.

Methodological approach

Exploiting the different chromatographic retention effects, carbon-normalized information of molecular size distribution, polarity, charge and aromaticity were obtained for DOC in the Arctic waters. Compounds separate based on their access to the pores of the chromatographic column. Retention therefore increases with decreasing hydrodynamic diameter of the molecule. Besides the size exclusion effect, secondary interactions with the surface of the stationary phase can influence the retention of

Acknowledgments

We thank Dr. Stefan Huber (DOC Labor, Karlsruhe, Germany) for the GPC-OCD analyses and for valuable discussions. We are also grateful to Ralph Engbrodt, Carmen Hartman, Benjamin Van Mooy and two anonymous reviewers for helpful discussions and constructive comments on the manuscript.

Associate editor: Dr. Edward Peltzer.

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